The present invention relates to communication between a telephone exchange, typically a private branch exchange, and telephone sets connected thereto.
In private branch exchanges, where communication between the exchange and the telephone sets connected thereto is performed in a digital way and by means of Serial communication, serial control information, which is sent, e.g., from a telephone set to the telephone exchange, must be converted to another suitable form, e.g., paraellel form, in order to be able to be processed by control circuits associated with the exchange and in certain cases the control logic of the telephone exchange itself. Also, the corresponding control information, which is issued from the telephone exchange, must be converted from the internal digital format of the exchange to serial signals on the line near the telephone sets connected to the exchange. Therefore, for each telephone set coupled to the exchange and its connecting line a line circuit is arranged performing conversion of the signals to a format suitable to the receiver. Such conversion circuits have been known for a long time as conventional construction elements and they perform conversion from digital signals in parallel form to digital signals forwarded in a serial form and the opposite.
The provision of a conversion unit for each telephone line may be rather costly, in particular considered from the viewpoint that each conversion unit in many cases is not used very often. An alternative solution to the conversion problem is then that a number of telephone sets share one common conversion unit. This solution can, however, result in problems, when several telephone sets in a group are to use, at the same time, the common conversion unit.
Prior conversion systems are described in U.S. Pat. No. 4,726,054; No. 4,325,147; No. 4,630,232; and No. 4,365,293.